The objective of this study is to evaluate the acute effects of a nonselective inhibitor of glutamate mediated synaptic transmission on the severity of parkinsonian signs and levodopa-associated motor response complications in patients with moderately advanced Parkinson's disease. In a controlled proof-of-principle clinical trial, the efficacy of the glutamate release inhibitor riluzole will be assessed through the use of validated motor function scales. Safety will be monitored by means of frequent clinical evaluations and laboratory tests.